weird scabs?

I went on my drive-around-aimlessly session for the day and I happened by a convenience store. The area around is pretty sketchy but busy enough that nothing really ever goes down..

Walking into the store, I passed this, I presume homeless fellow. He had deeply bloodshot eyes but acted sober and even politely nodded a "hello."

A moment inside to get a frosty beverage, vanilla coke, I was returning to the car and I noticed another guy give the homeless guy some change. Walking by, I asked if he needed change, he said yes, I gave him all the coins I had, at most 75 - 80 cents.

When I dropped the money into his hand, I noticed that all over his arms and legs were these oddly arranged scabs. Quite a few of them too. I've noticed these scabs several times now on others but I have absolutely no clue what causes them- I assume scratching though. But what the hell would be the leading cause of itching that warrants extreme scratching for homeless people? Is it drugs? Is it insects? Hygiene related? Viral? What causes it?

I've been thinking about it all. The reason? If it is a behavioral underlying cause, do I want to contribute to something that might be counter operative to growth? Both his and ours. Not really. I don't want to judge either. I just question my social responsibility under the various scenarios.